The writers may be still polishing up the script now. Whedon worked on his even while they were filming The Avengers, so it's not unusual nor is it a sign of impending doom.

Marvel also doesn't like to have scripts in circulation until the last possible moment. Feige and co. hate leaks. Not letting anyone see scripts until absolutely necessary prevents details from slipping out. It works, as Marvel had been pretty good at keeping major spoilers from leaking on most of its films.